1. Field of the Invention
This invention relates to pulse-echo acoustic ranging systems of the type in which a transmitter generates pulses of high frequency electrical energy at a predetermined frequency to cause an electro-acoustic transducer to generate shots of acoustic energy, and the same transducer is utilized to receive acoustic energy echoed from a target and convert such energy into electrical signals which are applied to a receiver.
2. Review of the Art
For effective operation of such a system both the transmitter and the receiver must be effectively matched to the transducer, and the input of the receiver must also be protected from high amplitude signals appearing at the output of the transmitter, which signals will be of much higher amplitude than the signals due to echoing of the transmitted acoustic energy. Since the piezo-electric transducers typically utilized are reactive devices, it is common to use transformers or other inductive components in the matching circuits to achieve some degree of tuning of the transducer to its operating frequency, thus increasing the Q or quality factor of the circuit. In this respect, circuit requirements tend to be different in different phases of operation. Whilst transmitter efficiency is favoured by a high Q, too high a Q results in delays in both the build up of amplitude of the "shot" of acoustic energy produced and, more importantly, extended high amplitude "ringing" of the transducer after cessation of the transmitter pulse. This ringing tends to limit the minimum range at which a target can be detected, and causes various difficulties in the recognition of echo signals reflected from a target. On the other hand, effective detection of weak and noisy long range echo signals is favoured by good impedance and noise matching to the receiver, although this is less important in the case of relatively high amplitude short range echo signals.
Various approaches to these problems have been proposed or used.
U.S. Pat. No. 3,613,068 (Thompson et al) utilizes separate receiver and transmitter transformers, with primary and secondary windings respectively connected in series with one another and with the transducer, the receiver winding being in parallel with back-to-back diodes, and a tertiary winding being provided on the transmitter transformer which shorts out the transmitter transformer secondary winding except during a transmit pulse. During a transmit pulse, the diodes limit the potential appearing across the primary of the receiver transformer, and also effectively take the receiver transformer out of circuit so far as the transmitter is concerned. The circuit requires two separate transformers, and a switching circuit for the transmitter transformer secondary. Since this switching circuit is controlled by the signal amplitude in the transmitter transformer, ringing of the transducer must result in some uncertainty as to point at which the relay performing the switching will drop out and remove the transmitter transformer secondary from the transducer circuit.
U.S. Pat. No. 4,199,2464 (Muggli) utilizes a single transformer connected to the transmitter, with the receiver signal being taken from a tap on the secondary of the transmitter transformer, which forms part of a variable Q filter. It is a feature of the Muggli patent that a frequency-varying pulse is utilized, and the variable Q filter permits the bandwidth of the circuit to be increased and its Q lowered during transmission and the receiving of short range echoes, and the bandwidth to be narrowed and the Q increased while receiving longer range echoes, under control of an external control circuit.
U.S. Pat. No. 4,785,429 (Folwell et al) utilizes back-to-back diodes in series with a winding of a transformer, in turn in series with a transducer, with a feedback circuit providing variable bias at the input to a receiver.
U.S. Pat. No. 4,701,893 (Muller et al) connects a transmitter and a receiver to different windings of a transformer, and utilizes a blanking signal to apply heavy damping to the transmitter winding following a pulse.
U.S. Pat. No. 4,597,068 (Miller) utilizes a common inductor for both transmission and reception, and controls a transmit pulse by energizing first a positive feedback amplifier and then a negative feedback amplifier to introduce and then remove energy from the inductive circuit.
U.S. Pat. No. 4,326,273 (Vancha) utilizes separate transmit and receive transformers, with back-to-back diodes connected in parallel with the secondary of the receive transformer through a potentiometer. The secondary of the transmit transformer and the primary of the receive transformer are connected in parallel, and the primary of the transmit transformer is only in circuit during transmission of the pulse.
U.S. Pat. No. 4,353,004 (Kleinschmidt) utilizes a pair of back-to--back diodes to shunt out part of a series resonant circuit during a transmit pulse, thus preventing it from short-circuiting the transducer, whilst the diodes cease to conduct during reception of low amplitude received signals, thus permitting the series resonant circuit to be functional to enhance efficiency during reception.
U.S. Pat. No. 4,114,467 (Thun) discloses, in FIG. 3, the use of a transformer having separate transmitter and receiver windings. Diodes associated with the receiver winding are switching diodes utilized to isolate the receiver during a blanking period. The transmitter and receiver windings are connected neither in series nor in parallel.
Japanese Published Application 58-206989 utilizes a varicap diode in a tuned input circuit to a receiver so that this circuit is detuned by high amplitude signals, thus reducing transfer of signals to the receiver input.